34 BULLETIN 61, U. S. DEPARTMENT OF AGRICULTUKE. 



area of the sink; the second, on the outside margin. In the first boring borax is 

 shown in percentages ranging from 1 to 5.5, and at depths extending from 18 to 50 

 feet. Below tliis depth traces of borax occur, while in the material extending from 

 the surface to 18 feet practically no borax, or only a trace, was found. In the second, 

 borax was reported in percentages ranging from 5.56 to 6.57 and extending from a 

 depth of 30 to 45 feet. Above and below these depths only traces of borax were 

 reported. The results of these borings would indicate that in some of the playas, at 

 least, might be expected concentration of borax minerals at depth. 



The minerals reported from deposits of the playa tv'pe are borax (sodium borate), 

 ulexite (sodium calcium borate), and colemauite (calcium borate). The content of 

 the crude material ranges from 5 to 20 per cent boric acid. Deposits of this type 

 are no longer of economic importance. 



Deposits of the second type are the important sources of borate minerals. In the 

 basin region they are worked at Ryan and in the vicinity of Furnace Creek (Mount 

 Blanco). For a description of these deposits the reader is referred to Campbell* 

 and to Keyes.^ 



These deposits are of importance in that they account for the source of a part, and 

 perhaps the principal part, of the borate material in certain playa deposits, notably 

 those in Death Valley and along the Amargosa River. Of the bedded deposits Keyes 

 describes two general types. The one is illustrated by the mines northwest of Dag- 

 get, Cal. In this locality the borate minerals are found in a finely divided state in 

 beds of blue clay. The workable beds contain from 10 to 12 per cent boric acid. 

 The other type is characterized by nodules and masses of almost pure colemanite in 

 clays and shales of Tertiary age. Respecting the origin of these deposits, Charles 

 Laurence Baker, ^ after discussing the possible formation of the deposits by the evapo- 

 ration of a body of water of considerable depth, presents the following: 



"The alternative hj'po thesis that these minerals had their immediate origin in hot 

 springs and solfataras opening directly into shallow lakes, perhaps only of seasonal 

 duration, or in playas, has much to commend it, especially when considered in 

 connection with the numerous evidences of shallow water deposition. These evi- 

 dences comprise ripple marks, sun cracks, and rain prints, which are foimd on the 

 finer as well as the coarser beds, and the layers of finer breccia and conglomerates 

 interbedded with the fine shales and tuffs. Shallow lakes or ponds probably existed 

 at times during the deposition of the fossiliferous tuff member, for they seem to be 

 necessary to account for the presence of the gasteropods. The paucity or absence of 

 fossils in the borate and the fine ashy and shaly tuff members, as well as the presence 

 of the colemanite, limestone, and gypsum layers, apparently indicates the salinity 

 of the waters. 



"There was great volcanic activity before and during the deposition of the Rosa- 

 mond. The larger fragments of lava were most probably derived from flows subject 

 to erosion somewhere in the area tributary to the basin of deposition. Interbedded 

 flows of both' acidic and basic lavas make up a part of the Rosamond. But the fine 

 volcanic ash was probably blown in by the wind or settled during explosive volcanic 

 outbm-sts and need not have come from the immediate vicinity. The common view 

 of the origin of calcium borate from solfataras and hot springs associated with the 

 abundant contemporaneous vulcanism is likely to prove to be the correct explanation 

 for the borax beds in this region." 



I am in accord with the main points in Baker's view. It must be noted, however, 

 that in the colemanite deposits of the type exemplified by Mount Blanco a consider- 

 able amount of secondary action is noticeable* There are many distinct veins of 

 colemanite associated with the layer beds. One gets the impression that these are any- 

 thing but bedded deposits. I am inclined to the view that the Mount Blanco deposits 

 represent the solution of the borate minerals from beds and their concentration in more 

 or less open fissures in close proximity as veins. 



ALUM. 



Spurr * reports a deposit of alum and sulphur 10 miles north of Silver Peak, Nev. 

 The area in which this mineral occurs is about 200 feet in diameter. The alum is asso- 

 ciated with sulphur in rhyolite. It forms a network of small veins in the broken 

 rhyolite. The rhyolite is intrusive in Tertiary sedimentaries. The mineral is a pure 

 potash alum, kalinite. Spurr considers that the alum results from the action of 

 steam and sulphuric acid emitted from solfataras. The acid attacks the potash and 

 alumina of the rhyolite and forms kalinite. 



1 Bui. No. 200, U. S. Oeoi. Survey. 



2 Borax deposits of the U. S., Trans. Am. Inst. Min. Enp., vol. 40, p. 674. 



3 Bulletin of the Depf . of Geolopy, Univ. of Cal., 6, p. 358. 

 ■< Professional Paper No. £.5, U. S. Geol. Survey, p. 157. 



